1. Field of the Invention
The present invention relates to the selection of an operating frequency for a hardness measurement system for measuring the hardness of a test object using a hardness sensor having a vibrator for applying vibrations to an object and a vibration detection sensor for detecting signals reflected from the object, and more particularly to a method for selecting the operating frequency and an operation frequency selection apparatus for the hardness measurement system for measuring the hardness of an object from frequency changes that occur according to the hardness of the object and including a phase shift circuit, which is connected in series with an amplifier to the hardness sensor, to change the frequency and to shift the phase difference to zero when a phase difference occurs between an input waveform to the vibrator and an output waveform from the vibration detection sensor.
2. Description of the Related Art
One method for measuring the hardness of living tissue includes pressing a probe against the tissue to be measured, applying vibrations, and obtaining a characteristic value corresponding to the hardness from frequency changes and phase changes by detecting the mechanical vibration response of the living tissue with respect to the input vibrations. In particular, as disclosed in Japanese Patent Laid-Open Publication No. Hei 9-145691, the present inventor has devised a hardness measurement system for measuring the hardness of an object from frequency changes that occur according to the hardness of the object by using a hardness sensor that includes a vibrator for applying vibrations to an object and a vibration detection sensor for detecting signals reflected from the object and a phase shift circuit, which is connected in series with an amplifier to the hardness sensor, to change the frequency and to shift the phase difference to zero when a phase difference occurs between an input waveform to the vibrator and an output waveform from the vibration detection sensor.
An example of a hardness measurement system using a phase shift circuit is shown in FIG. 8. In FIG. 8, a hardness measurement system 10 includes a hardness sensor 12 to be pressed against an object 8, such as living tissue, and a hardness detection part 20. The hardness sensor 12 includes a vibrator 14 for applying vibrations to the object 8 and a vibration detection sensor 16 for detecting signals reflected from the object 8. For this hardness sensor 12, two stacked piezoelectric elements can be used, with one used as the vibrator 14 and the other used as the vibration detection sensor 16. The hardness detection part 20 includes an appropriate DC blocking capacitor, an amplifier 22, and a phase shift circuit 24 connected in series between the output terminal from the vibration detection sensor 16 and the input terminal to the vibrator 14, a frequency deviation detector 26 for detecting the frequency deviation that occurs for compensating for the phase difference by the phase shift circuit 24, and a hardness converter 28 for converting the detected frequency deviation and outputting a hardness value. The frequency deviation detector 26 can use a general frequency counter and the hardness converter 28 can use a microcomputer to perform conversions using a pre-calibrated lookup table or perform conversion computations according to a predetermined conversion formula.
As described hereinabove, the phase shift circuit 24 is provided in a serially connected loop of—the vibration detection sensor 16—DC blocking capacitor—amplifier 22—vibrator 14—object 8—vibration detection sensor—, and has a function for changing the frequency and shifting the phase difference to zero when a phase difference occurs between the input waveform to the vibrator 14 and the output waveform from the vibration detection sensor 16. As shown in FIG. 9, it is preferable for the transfer-characteristic reference curve showing the amplitude characteristic and the phase characteristic with respect to the frequency of the phase shift circuit 24 to have maximum amplitude and invert a phase at the operating frequency ff. With the operating frequency ff as the resonance frequency, a circuit having this characteristic can be obtained by designing a band-pass filter where the amplitude gain is a maximum at the resonance frequency. More specifically, this circuit can be configured in hardware by the placement of electronic components or by implementing digital filter characteristics in software.
To describe the function of the phase shift circuit 24, a comparison can be made with a vibration system loop that does not include the phase shift circuit. Namely, the vibration system loop, formed from—the vibration detection sensor—DC blocking capacitor—amplifier—vibrator—object—vibration detection sensor—, forms a self oscillation circuit. Even when the hardness sensor 12 (vibrator 14+vibration detection sensor 16) is not in contact with the object 8, the self oscillation circuit is consisted with the space between the vibrator 14 and the vibration detection sensor 16 assuming the role as an object. And the overall system oscillates in a stable manner at some resonance frequency. Next, when the hardness sensor 12 (vibrator 14+vibration detection sensor 16) is in contact with the real object, the oscillation state of the overall system changes due to the influence of the mechanical vibration system for the real object. Namely, due to the magnitude of the spring constant expressing the hardness that describes the vibration system for the real object, a phase difference occurs or a change in frequency occurs. There are already many examples of the prior art of attempts to detect the frequency change and measure the hardness of objects. However, changes in the resonance frequency are usually extremely small so that precise detection is difficult. Moreover, there are few satisfactory measurement means for phase difference detection. The phase shift circuit 24 uses a transfer characteristic reference curve that expresses the amplitude characteristic and the phase characteristic with respect to frequency as described in FIG. 9, converts the change in phase to a change in frequency, and converts the difficulty to measure phase difference detection to an easiness to measure frequency measurement.
The effect will be described when the phase shift circuit 24 is provided in a serially connected loop of—the vibration detection sensor 16—DC blocking capacitor—amplifier 22—vibrator 14—object 8—vibration detection sensor 16—. When the phase shift circuit 24 is connected in the self oscillating loop that includes a real object, which has a mechanical vibration system, and an electrical oscillation circuit, the overall system operates so that the self oscillation is sustained under a condition which is called ‘velocity resonance’. ‘Velocity resonance’ refers a resonance having a maximum amplitude and a phase of zero at the resonance frequency. Namely, when the hardness sensor 12 (vibrator 14+vibration detection sensor 16) is not in contact with the real object 8, the operating point of the phase shift circuit 24 is determined so that the operation is stable at a frequency where the phase difference becomes zero between the input waveform to the vibrator 14 and the output waveform from the vibration detection sensor 16 with the space between the vibrator 14 and the vibration detection sensor 16 assuming the role as an object. This state is shown in FIG. 9 by frequency f1 and phase θ1.
Next, when the hardness sensor 12 (vibrator 14+vibration detection sensor 16) is in contact with the real object 8, a phase difference occurs between the input waveform to the vibrator 14 and the output waveform from the vibration detection sensor 16 due to the mechanical vibration system for the real object 8, namely, the magnitude of the spring constant expressing the hardness. Now, if the phase difference of only Δθ occurs due to the hardness of the object 8, the operating point of the phase shift circuit 24 shifts so that the self oscillation is sustained by ‘the velocity resonance’, namely, so that the phase difference Δθ is compensated and the phase difference of the overall system becomes zero. As described by FIG. 9, the operating point of phase θ1 and frequency f1 shifts to the operating point of phase θ1+Δθ and frequency f1+Δf, and with the phase difference of the overall system here at zero, ‘the velocity resonance’ is sustained and stable.
Namely, by connecting the phase shift circuit 24 in series within the loop formed—the vibration detection sensor 16—DC blocking capacitor—amplifier 22—vibrator 14—object 8—vibration detection sensor 16—, a phase difference Δθ compensation necessary for sustaining ‘the velocity resonance’ is performed. Simultaneously, the magnitude of the phase difference Δθ for which compensation was performed can be converted to the frequency deviation Δf. The frequency deviation Δf that is obtained here is not the amount of change in the resonance frequency as in the prior art but the result of converting the amount of phase change into an amount of frequency change according to the transfer characteristic reference curve for the phase shift circuit 24. Thus, the conversion coefficient Δf/Δθ can have an arbitrary magnitude depending on the design of the reference transfer characteristic curve for the phase shift circuit 24. Namely, a small phase difference can be converted to a large frequency deviation or an excessively large phase difference can be converted to a frequency deviation having an appropriate magnitude.
The frequency deviation obtained in this manner can be measured by an appropriate frequency counter and then converted to a hardness value on the basis of a predetermined frequency deviation versus hardness calibration relationship.